Cascading Style Sheets/Browser Extensions

Each web browser's layout engine has its own extension for the features of CSS it supports, whether they be proprietary, experimental or purely internal. For instance, many browsers only have experimental CSS3 implementations, so the CSS property is often prefixed by the layout engine's name. The most famous example would probably be border-radius, which for the longest time was only supported by WebKit and Gecko as -webkit-border-radius and -moz-border-radius respectively (border-radius is now supported by WebKit, Gecko, Presto & Trident without the vendor prefix).